High-Frequency EPR Studies of New 2p–3d Complexes Based on a Triazolyl-Substituted Nitronyl Nitroxide Radical: The Role of Exchange Anisotropy in a Cu–Radical System

Abstract

Using the 1-(m-tolyl)-1H-1,2,3-triazole-4-(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) (TlTrzNIT) radical and metal β-diketonate complexes [M­(hfac)2(H2O)2], where hfac is hexafluoroacetylacetonato, three new 2p–3d heterospin complexes were synthesized. Their structures were solved using single crystal X-ray diffraction data, and magnetic investigation was performed by DC and AC measurements and multifrequency EPR spectroscopy. Compounds 1 and 2 are isostructural complexes with molecular formula [M3(TlTrzNIT)2(hfac)6] (MII = Mn or Cu) while compound 3 is the mononuclear [Co­(TlTrzNIT)­(hfac)2] complex. In all complexes, the radical acts as a bidentate ligand through the oxygen atom of the nitroxide moiety and the nitrogen atom from the triazole group. Furthermore, in compounds 1 and 2, the TlTrzNIT is bridge-coordinated between two metal centers, leading to the formation of trinuclear complexes. The fitting of the static magnetic behavior reveals antiferromagnetic and ferromagnetic intramolecular interactions for complexes 1 and 2, respectively. The EPR spectra of 1 are well described by an isolated ferrimagnetic S = 13/2 (= 5/2 – 1/2 + 5/2 – 1/2 + 5/2) ground state with a biaxial zero-field splitting (ZFS) interaction characterized, respectively, by 2nd order axial and rhombic parameters, D and E, such that E/D is close to the maximum of 0.33. Meanwhile, EPR spectra for 2 are explained in terms of a ferromagnetic model with weakly anisotropic Cu–radical exchange interactions, giving rise to an isolated S = 5/2 (= 5 × 1/2) ground state with both an anisotropic g tensor and a weak ZFS interaction. Complex 2 represents one of only a few examples of Cu–radical moieties with measurable exchange anisotropy.

Two New Cobalt(II) Heterotrispin Dimers Bridged by Functional Nitronyl Nitroxides: Structure and Magnetic Properties

“…These results would seem to suggest that the interaction between spin carriers depends on the Co-O (radical) bond length and dihedral angle between basal plane and ONCNO unit [38][39][40][41][42]. The studies illustrated that even a minor modification of a ligand leads to significant changes in the molecular and crystal structures of the resulting Co (II) -nitronyl nitroxide complexes, and this maybe strongly affects the parameters of magnetic transitions as well as can result in the assembly of phases that cannot exhibit any magneto-structural anomalies.…”

Synthesis, Structure and Magnetic Property of a New Cobalt (II) Complex Based on Pyridyl-substituted Imino Nitroxide Radical

“…The design of magnetic compounds arising from the combination of transition metal ions with nitroxide radicals is steadily increasing [1][2][3][4]. Nitroxide radicals as stable organic radicals, have been widely used as molecular units in the construction of magnetic materials because their donor atoms can assemble an extended coordination geometry with changing magnetic coupling, giving rise to new functional materials with a variety of structural topologies [5][6][7].…”